CLI does not support reading from pipes

[lordmauve]

I tried using pipes to preprocess input files prior to consumption by Waiter.

waiter create --json <(jsonc2json deploy/waiter/template.json) --context <(jsonc2json deploy/waiter/prod.json) --context.version 5805415c55c19322c9b8da8595c3c0bb63e09a36

This fails with the error message

Unable to load JSON from /dev/fd/63.

This appears to be due to a redundant os.path.isfile() check here in waiter.data_format.load_file(). It would be better to use the EAFP pattern and catch/suppress/reraise FileNotFoundError and IsADirectoryError from the open().